In order to improve the degradation of communication quality due to drastic increases in mobile traffic in recent years and achieve faster communication, the standardization of Carrier Aggregation (CA) functions that enable a radio terminal (User Equipment (UE)) to communicate with a radio base station (eNode B (eNB)) by using a plurality of cells has been undertaken in the 3GPP Long Term Evolution (LTE). Note that the cells that a UE (User Equipment) can use in CA are limited to a plurality of cells of one eNB (i.e., a plurality of cells served by one eNB).
The cells that are used by a UE in CA are categorized into a Primary Cell (PCell) that has already been used as a serving cell when the CA is started and a Secondary Cell(s) (SCell(s)) that is used in addition to the PCell or in dependence thereon. Each SCell can be used by a UE as the need arises, and the use of them can be stopped. Note that starting the use of an SCell is called “activating” or “activation”. Similarly, stopping the use of an SCell is called “deactivating” or “deactivation”. Non-Access Stratum (NAS) mobility information, security information (security input) and the like are transmitted and received through a PCell during radio connection (re)-establishment (RRC connection Establishment/Re-establishment) (see Non-patent Literature 1). A downlink (DL) Carrier and an uplink (UL) Carrier corresponding to a PCell are called “DL Primary Component Carrier (PCC)” and “UL PCC”, respectively. Similarly, a DL Carrier and a UL Carrier corresponding to a SCell are called “DL Secondary Component Carrier (SCC)” and “UL SCC”, respectively.
A radio link recovery procedure that is performed when radio link disconnection (Radio Link Failure (RLF)) occurs in a radio link in a PCell during downlink data (DL data) transmission in CA is explained with reference to FIG. 10 (Non-patent Literature 2). Here, it is assumed that a UE uses a first cell (Cell1) served by an eNB as a PCell and uses a second cell (Cell2) as an SCell.
In steps S1 and S2, the eNB transmits DL data to the UE by using the PCell (Cell1) and the SCell (Cell2). In a step S3, the quality of the radio link in the PCell deteriorates and the DL data transmission from the eNB to the UE fails. In a step S4, the UE detects the RLF in the PCell (Cell1). In a step S5, the UE transmits a request for the reconnection of the radio link in the PCell (Cell1) (RRC Connection Reestablishment Request). In a step S6, the UE releases the SCell (Cell2) (SCell (Cell2) release). In a step S7, the eNB transmits a response to the reconnection request through the PCell (Cell1) (RRC Connection Reestablishment). In a step S8, the UE transmits a report about the completion of the reconnection through the PCell (Cell1) (RRC Connection Reestablishment Complete). As a result, the UE can receive DL data in the Cell1 again. In a step S9, the eNB transmits DL data to the UE by using the PCell (Cell1).
In FIG. 10, an example in which the UE detects the RLF is shown. However, when the eNB can detect the RLF before the UE does, the eNB may trigger the reconnection. As described above, in ordinary CA, the UE or the eNB can detect an RLF occurring in the PCell and reestablish the radio link connection. Accordingly, the eNB and the UE can resume data transmission, thus making it possible to minimize packet losses and the like caused by RLFs in the PCell. Note that, when the SCell (Cell2) needs to be used again after the completion of the reconnection, the eNB transmits configuration information for the SCell to the UE (RRC Connection Reconfiguration including SCell configuration) and also transmits to the UE a message indicating start of using the SCell (called “Activation”).